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240VAC single phase from three-phase? - Page 7

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Posted by Josepi on April 8, 2010, 3:09 am
 


There is another term for it. "Bi-phase supply"

This is all OT as the OP asked about a two phase 120/208 supply usage...LOL


====snip====


 I'm getting heartily sick of the confusion over what is normally
referred to as a "Bi-Phase" supply. In the UK such supplies are normally
only seen on building sites where a 55-0-55 vac supply is used to power
tools in order to limit the maximum rms voltage with respect to ground
to that of any of the two anti-phase lives (or hots) thus greatly
reducing the electrocution hazard.

 The tools are rated for 110 volt operation and receive exactly this by
using the two lives (hots). Effectively the supply is derived from a
centre tapped 110v secondary where the centre tap is earthed to prevent
an earth fault that would otherwise cause the voltage on the other leg
rising to 110v if the secondary were left 'floating'.

 The stateside domestic dwelling supply is simply the same scheme but
with double the voltage to feed high wattage loads and the centre tap
connected to the neutral return (which is tied to the local substation
earth). Lighting circuits and low wattage appliances make use of just
one of the bi-phase lives (hots) and the neutral.

 This has the advantage of a reduced electrocution risk and, as far as
GLS tungsten filament lamps are concerned, allow a more robust filament
for any given wattage to be used compared to the UK and european
standards of 240 and 220 volts respectively[1].

 Essentially, for a given wattage, a 115v lamp can use half the filament
length at twice the CSA compared to its 230v counterpart. This gives the
choice of either the same life with a higher efficiency or a longer life
at the same efficiency compared to a 230 volt lamp.

[1] Although the EC have 'Harmonised'(tm) the domestic PSU voltage
levels to a nominal "230" volt, the allowable tolerances in each country
of the EC have simply been adjusted to avoid the need to actually make
any change to the original 240 and 220 volt supplies.

 Most appliances designed for a nominal 230volt supply will work
perfectly fine on either voltage but tungsten filament lamps are an
exception to this rule since they are extremely sensitive to the effects
of voltage variation on their service life and efficiency and are
therefore designed for the voltage used in the region they are marketed
in. In this regard, the modern electronically ballasted CFL has the edge
over the traditional tungsten filament GLS lamp.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.



Posted by You on April 7, 2010, 5:31 pm
 




The above is EXACTLY why you don't understand what is REALLY GOING ON...
If you had just spent a little time listening in EE 101, instead of
sleeping, you would know just where your assumptions above, are leading
you down a Blind Alley, and why thew rest of us are laughing at your
ignorance.....

Posted by Josepi on April 7, 2010, 6:40 pm
 

Nobody is laughing. I can see what he is saying. Not everybody has studied
electrical vestors / phasors and this is very abstract for some.

His analogy on the batteries was an excellent one to help everybody
understand. Just a small detail was missed.

Your exagerated, over the top, personal attack doesn't help him at all. We
have dealt with this behaviour from another pair of hot heads (W+G) here for
years now and it has destroyed these groups, somehwat.

Note: Single phase on the transformer refers to the primary connection.


The above is EXACTLY why you don't understand what is REALLY GOING ON...
If you had just spent a little time listening in EE 101, instead of
sleeping, you would know just where your assumptions above, are leading
you down a Blind Alley, and why thew rest of us are laughing at your
ignorance.....



Posted by m II on April 7, 2010, 7:32 am
 


Before some nit picker finds my 'mistakes',

 > There is 120 degrees between the rms peaks of the sine waves.

'rms peaks' should have read 'peaks'.



 > 180^2 + 103.92^2 =  43009
 > square root of 14399.36  = 208 volts.


should have read:

square root of 43009   = 208 volts


mike



Posted by m II on April 1, 2010, 10:43 pm
 

Bruce in alaska wrote:


I have and I did. Many years ago.

Take two identical cars at the same speed having a head on collision.

The total vector force (mass X velocity) of both cars becomes ZERO.

That is what happens when you have a 180 degree difference between two.


Now, for electrical stuff. The proof of what I say may be found in the
common, or neutral conductor. If both hot sides are feeding identical
loads, the neutral of the circuit is carrying zero amperes.

The neutral carries only the *difference* in load currents.

If the voltage polarities were 180 apart, the neutral would carry the
SUM of the load currents. It doesn't.

Check out the Edison three wire circuit. It's applicable to batteries as
well as 240/120 volt residential distribution.

What you two are confusing is the sum of the current flows in the
neutral and the flow in the sources. The sources are IN PHASE and at 0
degrees.



===============================================
Vector Summation.—This is a simple geometrical process for ascertaining
the pressure at the free terminals of alternating current circuits. The
following laws should be carefully noted:

If two alternating pressures which **agree in phase are connected
together in series**, the voltage at the free terminals of the circuit
will be equal to their arithmetical sum, as in the case of direct currents.

http://www.meekmark.com/dp/Hawkins7/projectID421e9ab940c97.html

================================================


Please look at figs 2,125 to 2,128 and duplicate the numbers with a
ruler, pencil and protractor. The numbers don't lie.

The magnitude of the resultant voltage INCREASES as the angle of the
phase difference between two vectors DECREASES. Zero phase angle
difference gives the highest possible resultant voltage.



mike












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